Make sure you're working on ankle flexibility too. That will help you stay upright more than the hip flexibility - you won't be forced to sit your hips back so much, which is what forces you to lean forward.

As the weight increases, so does that hip-back, forward-lean position - I would add lots of OHS, snatch balance, Sots press, etc, making you do every rep as upright and deep possible to increase your comfort and strength in the bottom position.

Also muscle snatch, high-pull, tall snatch, and dip snatch to work on that speed under the bar, but also the mechanics - you need to do more pulling down and get more accurate with the placement of the bar overhead.